In order to achieve a remarkable economy in the preparation of aqueous dispersions of rubbery polymers, in general the polymerization of the monomers is carried out at rather high rates corresponding to relatively short polymerization times (e.g., from 6 to 12 hours). By this method, there are obtained latexes whose particles have a mean diameter below 2,000 A (in general between 500 and 1,200 A); the latexes of this type, having a solids content equal to or greater than 60% by weight, have the disadvantage, among others, of a high viscosity with consequent difficulty in handling.
Moreover, it is known that in some applications, for instance in the production of ABS resins, the latexes with polymeric particles below 2,000 A do not give satisfactory results inasmuch as the ABS resins obtained therefrom do not show good characteristics of impact resistance and superficial aspect, for which it is necessary to have at disposal, as an intermediate, a polybutadiene latex whose particles, at least in part, will have a mean diameter of not less than 2,000 A, and preferably between 3,000 and 7,000 A. To obtain such a latex there are used techniques that are characterized by low reaction rates and rather long polymerization times (from 40 to 70 hours).
In order to avoid such burdensome techniques, it has been suggested to conduct the polymerization in a short time and to increase the diameter of a portion of the polymeric particles by means of a suitable agglomeration process which, in general, is carried out after the polymerization, but which may also be carried out towards the end of the polymerization.
In order to increase the diameter of at least a portion of the polymeric particles of a latex there have been devised various techniques, such as for instance:
freezing;
the passage of the latex, under pressure, through suitable orifices of a very small diameter;
addition of electrolytes, organic solvents or water-soluble polymers such as polyvinyl alcohol, polyvinylmethylether, polymers of alkylene oxides, and so on, or the addition of latexes of copolymers containing a small percentage of ethylenically unsaturated monomers forming water-soluble homopolymers.
Those techniques have considerable disadvantages, such as a high consumption of energy (for instance in the case of the freezing technique), the probable formation of heavy quantities of coagulum, a rather low limit of the maximum size of the agglomerated polymeric particles, a strong dependence of the agglomeration on temperature and time, mechanical instability of the agglomerated latex, and difficulties in controlling and reproducing the distribution of the size of the agglomerated particles.